Add Intel encodings for brz.b1 and brnz.b1.

Use these encodings to test trapz.b1 and trapnz.b1.

When a b1 value is stored in a register, only the low 8 bits are valid.
This is so we can use the various setCC instructions to generate the b1
registers.

cranelift/filetests/isa/intel/legalize-custom.cton

@@ -28,3 +28,27 @@ ebb0(v1: i32):
     ; check: $new:
     ; nextln: return
 }
+
+function %cond_trap_b1(i32) {
+ebb0(v1: i32):
+    v2 = icmp_imm eq v1, 6
+    trapz v2
+    return
+    ; check: $ebb0($v1: i32):
+    ; check: brnz $v2, $(new=$EBB)
+    ; nextln: trap
+    ; check: $new:
+    ; nextln: return
+}
+
+function %cond_trap2_b1(i32) {
+ebb0(v1: i32):
+    v2 = icmp_imm eq v1, 6
+    trapnz v2
+    return
+    ; check: $ebb0($v1: i32):
+    ; check: brz $v2, $(new=$EBB)
+    ; nextln: trap
+    ; check: $new:
+    ; nextln: return
+}